1) Field of the Invention
The invention relates to a coating agent composition for coating aluminum which is based on vinyl ester copolymers.
2 ) Background Art
Many things in daily life, especially in the foodstuffs, cosmetics and medicines sector, are packaged with flexible materials. These packaging materials also include aluminum foil to a large extent. This can be used by itself or in combination with other substances, such as plastics (PE, PP, PS, PVC, PET and the like, glass or cardboard. To seal the materials mentioned with the aluminum foil, for example, the heat-sealing method is used, which means that the materials are firmly bonded to one another by applying elevated temperature and pressure. However, since untreated aluminum foil is not sealable, it is coated with a polymer film, a heat-sealing coating or else a heat-sealing adhesive. During the sealing operation, the polymer becomes soft due to the elevated temperature and, in conjunction with the pressing pressure applied, ensures, in the cooled state, bonding of the materials to be sealed. A heat-sealing coating should, therefore, have a good adhesion to aluminum foil and should be transparent; it should be possible to paint over it, it should be readily sealable with itself and other materials, storage-stable and physiologically acceptable, and it should have a glass transition temperature of not more than 100.degree. C. and at the same time the highest possible block point in order to avoid sticking of the coated aluminum foil.
Polyacrylates or polymethacrylates such as are described in EP-A1 417 570 (CA-A 2025368) or EP-A2 316 753, and chlorine-containing polymers such as are known from DE-C2 29 06 118 (U.S. Pat. No. 4,267,094) are employed as heat-sealing coatings for coating aluminum foil. A disadvantage of the polyacrylates and polymethacrylates is their generally poor adhesion to aluminum. When these polymers are used, the aluminum foil must be primed beforehand in most cases. Chlorine-containing copolymers are as a rule employed as primers, so that chlorine-containing products, either by themselves or in combination with acrylate systems, are currently used in most cases for laminating aluminum foil. Vinyl chloride/vinyl acetate/vinyl alcohol tarpolymers (DE-OS 28 38 383=U.S. Pat. No. 4,141,870) and vinyl chloride graft copolymers (EP-A-117 350) are known from the literature for coating aluminum. Recently, however, the use of chlorine-containing products in short-lived packaging material, above all in the foodstuffs sector, has been avoided as far as possible. There is accordingly an urgent need for chlorine-free polymers for coating aluminum, specifically for finishing aluminum foil.
There was, then, the object of providing a coating agent composition specifically for finishing aluminum foil, which neither comprises chlorine-containing polymers nor has to be applied using a chlorine-containing primer. The polymers on which the coating agent is based should therefore have both a good adhesion to aluminum and a good sealing seam strength, in the dry and in the wet state, for use as a heat-sealing coating.